Energy balance and solar heating in Neptune's upper atmosphere.

Abstract

We have constructed the thermal structure of Neptune's stratosphere and low thermosphere with pressures between 10⁻³ μbar and 100 mbar using a radiative-conductive model which includes solar UV and EUV heating, non-LTE cooling by hydrocarbon fundamental bands, cooling by H₂ collisional induced opacities, and heating by the CH₄ near and far infrared bands. We have thoroughly investigated the availabilities of different techniques in modeling the CH₄ near-IR bands (3.3, 2.3, and 1.7 μm) and calculating the heating rates of these bands for pressures between 10⁻³ μbar and 100 mbar and temperatures between 50 K to 300 K. We have established an accurate and efficient way which is a combined method of correlated-k model and the Baines et al. (1993) empirical model to calculate these heating rates. The same method can also be applied to any other atmosphere of a Jovian planet. Through comparing the calculated temperature profiles and the measured one of Neptune's upper atmosphere, we have set constraints on the magnitudes, locations and the regions those are extended by for the stratospheric aerosol heating, heating by the source located in the mesosphere and heating by the conducted flux from the thermosphere. We also found that by using a constant CH₄ mixing ratio in the stratosphere of Neptune, 1.3 x 10⁻³, obtained by Yelle et al. (1993) through analyzing Voyager solar occultation data, the measured stratospheric temperatures between 20 to 100 mbar can be best matched by the calculational results.